Cyclic 3',5'-nucleotides, such as cyclic AMP and cyclic GMP, are produced intracellularly in response to hormone binding at membrane receptor sites, and these cyclic nucleotides activate protein kinases which eventually affect metabolic processes. The only known fate for cyclic nucleotides is their hydrolysis to 5'-nucleotides by phosphodiesterases. The objective of this project is to obtain sufficient amounts of a pure cyclic nucleotide phosphodiesterase to characterize some of its chemical and structural features. Soluble diesterases from swine heart and kidney and beef heart are being purified by affinity chromatography and preparative electrophoresis. Comparisons of molecular weights, subunits, amino acid composition and amino-terminal residues are being sought. The role of a protein activator in these tissues is also being investigated.